Both of these are excellent machines. You simply cannot go wrong with either. Nor can you go based on what others preferences are. The very best way to determine which is best for you is to think about what you are going to use it for. What your needs are as opposed to your gut feeling based on attraction to "the look".
I've chosen the 13".
For me screen size is of great importance, because even though this is my air travel laptop, (companion to my 15" MBP) I will still be spending a lot of time on it. In my case there's no substitute for reducing the amount of scrolling, and the ability to work with multiple windows open.
Your needs are probably quite different, so take your time, perhaps you may want to write a list of pros and cons for each size. At the end of the day, choosing what's right for you, will make a big difference.